FAQ

Below are some frequently asked questions.

1. What is POLY-CRETE™ mineral composite?

POLY-CRETE mineral composite is a formulation of a high strength epoxy resin system, mixed with scientifically blended quartz aggregate fillers. In addition, the system includes chemical additives and curing agents to improve the strength and durability of the composite.

2. How good is the chemical resistance?

The quartz aggregate is 99.6% pure silicon dioxide and offers excellent strength and chemical resistance. The epoxy binder offers excellent resistance to most common solvents, acids, alkalis and cutting fluids used by machines.

3. What are some design precautions?

Things to avoid are thin wall sections, negative draft angles where tooling must be drawn against the part to be extracted, high tensile loading, shock loading, tapped holes too close to the edge and underside horizontal surfaces formed by the tooling, thus causing air to be trapped and creating surface voids.

4. What is the minimum wall thickness?

The standard POLY-CRETE mineral composite mixture has aggregate up to 5/8" in size. The minimum section thickness should be at least two times the aggregate size or 1.25". Thinner sections can be cast by changing the aggregate size and formulation. The overall casting design must be evaluated for thinner sections, and all applied forces must be analyzed.

One of the major advantages of using POLY-CRETE mineral composite castings is the ability to cast sections of varying wall thickness without causing internal stresses. Metallic castings require consistent cross section thickness to reduce internal stresses and distortions.

5. What size parts can be cast?

To utilize the standard POLY-CRETE mineral composite formulation the part needs to be at least as large as a bread box, or about 30 lbs. The maximum size part is limited only by the ability to handle and ship the part. The average size part weighs between 800 and 2,000 lbs. Smaller parts can be cast using a special POLY-CRETE mineral composite mixture.

6. How precise can parts be cast?

Part flatness, hole location, hole diameter, and feature measurement are all measures of precision. The precision of the cast part is determined by mold design, mold construction and process variables. Using a properly designed, high quality steel mold, parts can be cast with a flatness of .002 in/ft, feature tolerances of .002 in/ft and hole diameters of +/- .005. When closer tolerances are required, or a low cost mold is to be used, tolerances can be achieved using secondary machining, lapping, grinding or replicating processes.
By eliminating secondary operations, the delivery time is expedited, reducing work-in-process inventory and allowing quicker response to customer delivery requirements. Deliveries can be days, instead of months as is typical for metallic castings.

7. How accurate can the overall cast height be held?

In most cases the height of the part is held within +/- 1/8". Parts are usually poured upside down, with the top of the mold open. This cast surface (the bottom of the part) is therefore very rough and not closely controlled. Where the overall part height is of concern, or when all surfaces must be finished, a secondary operation (post casting or machining) can be performed. This will add to the cost of the part. Another option is to cast pre-machined steel plates to achieve desirable results.

8. What can be done about the low tensile strength?

Most machine tools are subject to compressive loading. Where parts have tensile loading the section thickness should be increased to accommodate the loading. If the section thickness must be kept small, internal reinforcements should be used. These internal reinforcements can be structural rebar, threaded rod, angle iron, I-Beams, round or rectangle tubing, fiberglass or carbon fiber mat or rebar, or any other rigid structure.

9. Can POLY-CRETE™ mineral composite castings be painted?

Parts can be painted with any commercial paint, including new water base paints. Priming is not necessary as these parts will not rust. It is necessary to properly clean the surface to remove mold release from the part surface, or the paint will not stick. Surface imperfections can be filled using standard auto body filler.
Most POLY-CRETE mineral composite parts are not painted. Unlike metallic structures which rust or corrode, POLY-CRETE mineral composite castings will not rust or corrode and are impervious to most common solvents.
POLY-CRETE mineral composite castings are typically cast black, white or grey. It is possible to cast other colors, but this requires significant production volume and color matching is not possible. The part surface finish will exactly duplicate the mold surface, from high gloss, to matte to textured.

10. Does this material creep?

By nature, all materials creep. With proper part design POLY-CRETE mineral composite parts have one of the lowest creep rates of any construction material. Metallic parts not only creep, but also distort due to internal stresses. High precision parts should be produced from POLY-CRETE mineral composite. It is important that the POLY-CRETE mineral composite casting be properly designed and processed. Aggregate sizing and proper compacting is necessary to assure minimal air voids, along with solid aggregate contact, thus minimizing any creep.

11. How can I reduce weight?

POLY-CRETE mineral composite has a density of .084 lb./cu. in., approximately the same as aluminum. Because the wall thickness will be greater than an aluminum casting, the part weight will be higher. Part weight can be reduced by casting foam cores internally. This creates a closed box design that provides a stiffer design than open-end cores used in metallic castings. Weight reduction can also be achieved by using hollow structures such as PVC, carbon fiber or steel tubing. This not only reduces the weight of the part, but it also adds stiffness. Light-weight fillers can also be used, but this typically reduces the part strength.

12. How can the weight be increased?

The density can be increased by using high density fillers, such as iron flakes, ceramic beads or iron ores.

13. What do I do about dowel pins?

Dowel pin holes are cast directly into the POLY-CRETE mineral composite. The cast hole allows a perfect fit with a slight press fit. Pre-machined steel inserts can also be cast with the desired pin hole diameter.

14. How long does it take to cure?

Typically the castings stay in the mold overnight. A shorter cure rate is used for simple and smaller parts, where high volume production and low precision is required. For very complex, or high precision parts, depending on the desired as-cast accuracy the cure rate is slowed to 24 - 48 hours. This eliminates the exothermic heat, which can cause mold distortions. The casting's curing time depends also on the mass. A 10,000 lb casting will generate more exothermic reaction than a 200 lb. casting, thus curing faster. The cure time and rate can be adjusted by the hardener used.

15. What are threaded inserts made of?

Tapped holes are created by casting threaded inserts into position. These inserts are produced from steel, plated steel, stainless steel, brass and nylon. Tapped hole locations are determined by the mold, which means once a mold is qualified, subsequent part inspection is minimized.

16. How strong are the inserts?

In most instances, the insert is strong enough to allow a grade 8 bolt to be tightened to the proper torque. For extreme bolt loading, the inserts can be anchored deeper into the casting. The standard inserts have an external configuration to allow proper anchoring in the polymer composite castings. Small inserts (such as M3 or M6) should be kept at least one diameter from the part edge. Larger inserts used for lifting heavy parts must be strategically placed in the casting. POLY-CRETE mineral composite cannot be tapped. If a tapped hole is required after casting, a hole is drilled and an insert is epoxied into position using a template or fixture. If tapped holes need to be added during the assembly operation, steel pieces should be cast in the relative location. Steel plates can be cast below the surface to allow corrosion resistance or for a better appearance.
17. Can POLY-CRETE mineral composite castings be machined?

17. Can POLY-CRETE mineral composite castings be machined?

POLY-CRETE mineral composite castings can be ground using conventional aluminum oxide wheels, carbide wheels or diamond wheels and coolant to reduce the dust. The surface can also be machined using carbide cutting tools. Machining is more difficult and must be planned ahead so the surface can be prepared accordingly.

18. Should the edges have a radius?

Although radius edges are stronger, it is very difficult to produce steel tooling that will result in radius edges. Fiberglass molds can be used to produce radius edges on castings, or wax fillets can be applied during the mold preparation. Many tools use a combination of materials such as steel and aluminum. Aluminum castings are the choice for contoured surfaces.

19. Is poor heat transfer a problem?

Poor heat transfer allows the base to resist heat transfer. In the case of machine tool bases, poor heat transfer results in distortions due to heat build-up in one section, transferring to distort another section. This will not occur with POLY-CRETE mineral composite castings. Transient heat loads due to outside doors being opened or sun light coming through a window, will have little effect on the base. Metallic structures tend to distort quickly due to minor heat loads. Where heat transfer is important, vent holes or internal coolant lines can be cast in place.

20. How does heat affect the part's strength?

POLY-CRETE mineral composite is made from a thermal setting polymer, so it will not melt. Like any other common material, the strength decreases as the temperature increases. The strength is only slightly reduced up to 1600 F. Parts can be used up to 2500 F, where the strength will be approximately 50% of the room temperature strength. For higher temperature applications a different resin system can be used.

21. What is the cost per pound?

The cost per pound is not comparable to metallic castings as POLY-CRETE mineral composite polymer casting parts are cast to finished tolerances. Only the cost of finished parts should be compared. Metallic castings are quoted on a cost per pound, but they require secondary machining so the delivered cost per pound is much higher. Finished POLY-CRETE mineral composite parts sell for 30% less than metallic castings in most cases.

22. Is EMI shielding necessary?

In some instances supplemental EMI shielding is necessary. This can be achieved using an external shield, a cast-in internal shield or EMI paint. An EMI powder can be cast into the surface coat if painting is not an option.

23. What are some of the benefits of room temperature casting?

PVC piping can be cast in for wire ways, air venting or coolant flow. Oil lines can be cast in. Cardboard tubes can be used to reduce weight. Low friction way surfaces can be integrally cast. The room temperature casting process allows merging plastic and metal into the same casting.

24. How much does a mold cost?

Mold cost includes mold design and mold production. Mold design is often supplied free, but can run between $2,500 and $5,000 depending on the complexity. The mold design belongs to CPT; the mold belongs to the customer. A high quality steel mold typically costs between $5,000 and $150,000 depending on the size and complexity.

25. What material should I make the mold from?

Molds can be produced from steel plates, wood, sheet metal or fiberglass. Wood is used for prototype parts for quick delivery. Life expectancy is only a one part, although with careful handling several parts can be produced. Tolerances cannot be held, so secondary operations are necessary to obtain precision.
Fiberglass molds are used for high volume, low precision parts.
Sheet steel molds are used for lower cost, high volume parts.
Medium precision parts can be cast in a sheet steel mold, with the proper design.
High precision parts can only be cast using a heavy duty steel and aluminum mold.

It is often common to build hybrid tools by using Steel, Aluminum, wood and plastic materials.

26. How long will the mold last?

The mold life depends on the mold design, construction, and handing. A good quality steel mold should last hundreds of parts, with only minor maintenance.

27. Does the part need draft?

Draft is not necessary, but it can reduce production time (cast parts can be produced with no draft, negative draft or recesses.). High volume parts should be produced in a mold with draft. The tool construction cost is higher for molds with draft.
Draft should be 1o - 50 per side. Molds that have no draft require complete disassembly to extract the part and therefore have a higher production cost.

28. How energy efficient is the mineral casting process?

POLY-CRETE mineral composite casting process uses much less energy than metallic castings. To produce a metallic casting iron ore must be mined, smelted, melted for casting, and machined to tolerance. POLY-CRETE mineral composite resins use very little energy to produce and the casting process is done at room temperature. In addition, the mineral composite material once cured can be recycled or used as backfill and it poses no threat to the environment.